1. Field of the Invention
The present disclosure generally relates to the field of microelectronic circuits and more particularly but not exclusively an integrated electronic circuit comprising an adjustable resonator.
2. Description of the Related Art
Acoustic resonators are components that have been the subject of many studies. Traditionally, one distinguishes between Surface Acoustic Resonator (SAW) and Bulk Acoustic Resonators (BAW). In SAWs, the acoustic resonator is located on the surface of a semiconductor product while, in BAWs, it lays inside a volume delimited between a lower electrode and a higher electrode so that the acoustic wave develops in this volume.
Acoustic resonators are frequently used in radio frequency (RF) filtering and in particular in mobile telephony. They are likely, however, to be useful in many other applications in the future but the principal obstacle to the use of acoustic resonators lies in the difficulty of integrating them into a semiconductor product.
Indeed, the characteristics of the components issued from production lines show great dispersions, and even BAW-type resonators that are the one's that are best suited for arrangement on a silicon substrate. Even with particularly ambitious and inevitably expensive specifications—for example, with a tolerance of about 1 percent for resonator element dimensions, dispersion on electric characteristics of the resonator cannot be eliminated.
Such a dispersion problem is typically solved by selectively choosing components so as to retain only those products from the production lines that comply with precise specifications.
This approach cannot be chosen when aiming to integrate such an acoustic resonator on a substrate. Indeed, in this case, discarding a great number of manufactured products only because part of the product—which only accounts for a fraction of the added value of this product—does not show the characteristics specified in specifications, is quite out of the question.
This constitute a crippling obstacle to the direct integration of such acoustic components into an integrated circuit, this obstacle being likely to slow down the general use of acoustic resonator, this not only in RF applications.
The following documents illustrate background art:
Reference work “RF MEMS CIRCUIT DESIGN FOR WIRELESS COMMUNICATIONS”, Hector J De Los Santos, Artech House, ISBN 1-58033 329-9, 2002, p. 163 and following, comprise general information on BAW-type acoustic resonators. This work does not address the problem of BAW resonator integration into an integrated circuit.
The integration of BAW-type resonators into an integrated circuit has been considered in the document “FBAR FILTERS AT GHZ FREQUENCIES” by C. Vale, J. Rosenbaum, S. Horwitz, S. Krishnasvamy and R. Moore, in FORTY FOURTH ANNUAL SYMPOSIUM ONE FREQUENCY CONTROL, IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM, 1990. This document describes the use, on the same substrate, of FBAR-type components in combination with passive elements in order to make filtering circuits. This document does not address the problem of the integration of these components into a semiconductor product and moreover, it does not describe how to increase the precision of the manufacturing process to allow such integration.
U.S. Pat. No. 5,446,306 entitled “THIN FILM VOLTAGE-TUNED SEMICONDUCTOR BULK ACOUSTIC RESONATOR (SBAR)” discloses tuning of a BAW resonator by means of a d.c. voltage, but does not actually describe how to proceed in practical terms. Moreover, it completely ignores the issue of the integration of such resonators.
U.S. Pat. No. 5,714,917 entitled “DEVICE INCORPORATING A TUNABLE THIN FILM BULK ACOUSTIC RESONATOR FOR PERFORMING AMPLITUDE AND PHASE MODULATION” discloses a BAW-type resonator which is made adjustable in order to carry out phase and amplitude modulation. This document by no means describes how to adjust resonator frequencies nor does it show how to facilitate its integration into an integrated circuit while getting around the limitations inherent in the manufacturing process.
U.S. Pat. No. 2004/0033794 entitled “RESONATOR CONFIGURATION”, published on Feb. 19, 2004 discloses a process for tuning a resonator mounted on an integrated circuit by means of a second reference resonator also mounted on the same substrate. The reference resonator is used in a VCO-type (Voltage Control Oscillator) circuit to generate a local oscillation frequency. This patent does not indicate how to practically proceed to obtain a precise and effective tuning of the circuit using the resonator.
The French Patent Application No 0315480 (applicant reference 03-GR1-267) filed on Dec. 29, 2003 by the present applicant, entitled “Résonateur acoustique intégrable et procédé d'intégration d'un tel résonateur” and not published to the date of filing of the present application, describes a new acoustic component that can be easily integrated into a silicon substrate. This component is based on the use of an acoustic resonator of the BAW-type (Bulk Acoustic Wave) having two resonant frequencies and a high coefficient of quality. This resonator is associated with two judiciously selected partner elements, namely a first inductive element, variable or not variable, and a second capacitive element that is generally variable. The inductive element is set in the vicinity of said first and second resonant frequencies. The variability of both inductive and capacitive partner elements makes it possible to set the characteristics of the unit formed by the acoustic resonator associated with its two partner elements, and in particular resonant and antiresonant frequencies, and their respective quality factors.
A new tunable resonator element results, having a high quality coefficient as well as a remarkable aptitude for integration into a semiconductor product. Thus, it becomes possible to compensate for the lack of precision of the production line and the integration of particularly sophisticated circuits directly on the silicon substrate can be considered, and in particular a complete transmission—reception set in a single semiconductor product.
It remains that this new component, whose applications will increase and be diversified in the future, must be associated with an powerful control chain in order to fully benefit from the new possibilities it presents. Indeed, it is observed that while a discrete component varies little according to the production line (thanks to selection) or according to temperature (it is located on a distinct circuit), the case is very different when said component is arranged on a substrate with other electronic circuits and with whom it shares temperature variations.
The control chain of the new acoustic component comprising the BAW resonator will have not only to compensate for the insufficiencies related to the integrated resonator manufacturing process, but also to deal with the resonator's sensitivity to temperature.
Such is the purpose of this application.